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I had an argument with a work colleague. He claims that me cycling to and from work is less environmentally friendly than driving because

  1. I will eat more as a result of expending more energy.
  2. I have to take 2 showers a day (1 at work, 1 at home).

He claims that these are more wasteful than the fuel required to drive 25km every day.

Can anyone point me to any evidence for or against this claim?

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    IMO any answer is going to be difficult; we don't know what your diet consists of and whether or not you eat more vegetables than meats. We also don't know how long your showers are or how hot they are. The amount of pollution from driving a car can be roughly estimated, but again will depend on the make/model (gas/diesel), general condition of the engine, the inflation of the tires, etc. – Darwy Aug 21 '11 at 12:03
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    Another question is, whether he would normally perform some jogging after work, to stay fit, if he did not cycle, and whether a car is needed at all, if not used to go to work, and how much energy is spend into earning the money for car/for bycícle, and how this money is spent if not spent for driving. Driving 5 times a week 25 km=125km - can we assume 5 to 10l for fuel? Maybe calculating in kwH would be the easiest way of approximation? – user unknown Aug 21 '11 at 19:57
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    Is your friend telling you he never works out at the gym, requiring a second shower? Would he do those workouts if he was cycling? – DJClayworth Aug 22 '11 at 3:27
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    Who says that a cyclist will eat less? Maybe the car driver eats the same as the cyclist but puts on weight in the process... – Lagerbaer Aug 24 '11 at 0:29
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    I think the question is nonsense. Driving a bicycle requires energy to move 70kg (avg.human weight) + 15kg (bicycle weight) = 85kg. Driving a car requires to move some 2,000kg. I believe human digestion is about as efficient, as internal combustion engine. But at least to even these energies up, you need car engine to be some 24x times more efficient. – user4598 Aug 24 '11 at 11:25
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This calculation is pretty much worst-case for the bicycle. It doesn't take into account that food production has actually stored CO2 from the atmosphere which is now just released back, while fuel was originally underground and its CO2 is added to the carbon cycle. Neither does it take into account mercury/other emissions, nor the wear-and-tear in cars and bicycles, nor the enormous benefits if the bicyclist doesn't even own a car.

A vegetarian bicyclist certainly wins,

for a meat-eater it depends.

The question is what the bicyclist eats to get the required energy. Estimated from calculations below and this table:

  • WIN (1): chicken, milk, eggs, herring, tuna, farmed salmon or (just about) any vegetables
  • WIN (2): half vegetables, half some other fish or pork
  • WIN (3): 30% beef, 70% vegetables
  • LOSE: a lot of lamb or shrimp; (2) or (3) with too little vegetables

Note that the fractions above are of energy, not weight.

The cargo by Pulpolux !!! on Flickr Image by Pulpolux !!! on Flickr. CC-BY-NC 2.0


And here's the calculation. The initial assumption is that the choice between car and bicycle doesn't affect the amount of other exercise. In practice, it might be different, but this is the only way we can calculate.

Car

CO2 emissions for the average new car in 2006 were 167.2 g/km, so let's assume this amount. For 25km the car pollutes 4.18 kg CO2.

Bicycle

The extra food may be significant

Riding a bicycle uses up 1.62 kJ/(km∙kg). Let's assume the rider weighs 80kg, which is below or above average for a male depending on country. Allow 20kg for bike, clothes etc. and we have 100kg. Thus the 25km takes 4050kJ total -- that is 967 kcal.

Now, the food CO2 equivalent per kcal depends a lot on the type of food. For example:

  • soy 0.07g/kcal
  • chicken 1.67g/kcal
  • beef 13.82g/kcal

So, the CO2 equivalent from the extra food needed for cycling would be:

  • 0.068 kg for soy
  • 1.6 kg for chicken
  • 13.4 kg for beef

The extra shower doesn't really matter

The bicyclist also takes an extra shower. Let's use the same assumptions as a CBC campaign:

Assumes average shower time in Canada of 7.6 minutes (Source: Ipsos-Reid poll for GoBlue.org, 2008), average shower flow of 15 L per minute, and five showers per week. Also assumes 0.06 kWh electricity to heat one litre of water, and 22 g CO₂eq produced per kWh electricity used (Source: BC Hydro).

For 7.6 minutes the water usage is 114 liters, which requires 6.84kWh to heat. The CO2 equivalent from taking the shower is thus 0.15kg. This is just 3.6% of what the car is using so it's quite negligible.

The total

For most foods other than pork, beef or lamb, the total energy consumption is below what the car uses. For example, eating only chicken, the bicyclist's total would be 1.75kg, which is significantly less than the car.

The limit where the bicycle pollutes more than the car is if the food creates more than 4.17 g/kcal CO2 equivalents. So the bicyclist can easily afford to eat some beef, if the main energy source is vegetables.

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    I've tried to make all my assumptions explicit. For a calculation like this, there are bound to be many. I think the initial assumption says it all: if we exercise in some form anyway, there's just no way the car wins, ever. But this is more an opinion... – dancek Aug 22 '11 at 9:29
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    But the CO2 from food sources is part of a closed loop system and not at all comparable to CO2 from fuel use. CO2 for food was pulled from the environment while most automobile fuel is releasing CO2 that was sequestered underground. Better comparison for "worse for the environment" is CO2 for food transportation compared to CO2 for fuel use (plus CO2 for fuel transportation). – freiheit Aug 23 '11 at 18:32
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    And it is more or less a CO2-answer only, but the question is open for aspects of other pollutions (quicksilver, for instance). And in the big scale, a road for bicyclists, weighting 100kg each, is much less environment unfriendly, than a road for cars of 1, 2 or 3t. – user unknown Aug 23 '11 at 22:15
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    I agree this is a limited view. Feel free to edit in calculations of other aspects, especially if they don't require new assumptions to be made. I'm guessing that if we were to take everything into account, the car is very bad compared to the bicycle. My point here was more to take the worst-case scenario for the bicycle and point out that it still wins. – dancek Aug 26 '11 at 7:27
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    One minor point: according to wikipedia (en.wikipedia.org/wiki/Muscle) "The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%". So we really need to multiply the quantity of food, and thus the CO2 emissions, by about 5. This makes chicken comparable to the car, and meat even worse than it was. I guess this is a reflection of just how much CO2 is released in the transport of food to your dinner table. – tom Sep 30 '11 at 9:58
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One of the questions raised by several of the commenters was that while one can estimate the calories burned by bicycling, it is hard to estimate what change in calories the cyclist requires (over what would have burned by driving to work). I designed an experiment using a Heart Rate Monitor to help me estimate just that.

Notes and assumptions

Calories were determined by a Heart Rate Monitor, which infers calories burned based on heartbeat count. Assumption: number of heartbeats are directly proportional to actual calories burned.

Calories were burned either 1) While driving to a bus stop in a car and then taking the bus to or from work or 2) while bicycling to or from work. Assumption: the cyclist will eat more on the day he cycles, corresponding to the larger quantities of calories being burned.

The car travels mostly at highway speeds to the bus park-and-ride. Distance 5.7 miles. Assumption: Nominal 30 mpg and 1 passenger were used to estimate 0.16 gallons of unleaded fuel.

Using http://www.eia.doe.gov/oiaf/1605/coefficients.html; a car fueled with unleaded gasoline produces 8.87 kg of CO2 per gallon.

The bus takes a (mostly) express route over highway. Distance 10.8 miles.

Using http://docs.wri.org/wri_co2comm_2002_commuting_protected.xls and medium haul of .20 kg CO2 per passenger mile.

CO2 is a proxy for environmental impact.

The commuter is a vegan male, weight 68 kg (150 pounds).

Design of statistical test

A difference between means (of total CO2 production) was the chosen hypothesis test.

$\mu1 represents the mean CO2 emissions required to fuel the bus, car, and the subject's body while commuting.

$\mu2 represents the mean CO2 emissions required by the foods needed to sustain the subject's body while bicycling.

Null hypothesis: $\mu1 - $\mu2 <= 0

This corresponds to the OP's friend’s assertion that bicycling produces as much or less CO2 as driving.

Alternative hypothesis: $\mu1 – $\mu2 > 0

Will choose a significance level of 0.01 Will use a one-tailed test.

Experimental data

Measurements were taken over a period of about one month. There were 18 measurements taken: 9 by bus and car, 9 by bicycle. Here is the data:

Date    Mode    Cal             Food    Bus Car Total
                                CO2     CO2     CO2     CO2 
                                (kg)    (kg)    (kg)    (kg)
24-Aug  Bike    781             0.109   0.00    0.00    0.11
24-Aug  Bike    830             0.116   0.00    0.00    0.12
28-Aug  Bus/Car 177             0.025   2.16    1.42    3.60
30-Aug  Bus/Car 326             0.046   2.16    1.42    3.62
31-Aug  Bike   1148             0.161   0.00    0.00    0.16
31-Aug  Bike    770             0.108   0.00    0.00    0.11
11-Sep  Bus/Car 117             0.016   2.16    1.42    3.60
11-Sep  Bus/Car  77             0.011   2.16    1.42    3.59
12-Sep  Bike    842             0.118   0.00    0.00    0.12
12-Sep  Bike    802             0.112   0.00    0.00    0.11
13-Sep  Bus/Car 117             0.016   2.16    1.42    3.60
13-Sep  Bus/Car 102             0.014   2.16    1.42    3.59
17-Sep  Bus/Car 274             0.038   2.16    1.42    3.62
17-Sep  Bus/Car  97             0.014   2.16    1.42    3.59
20-Sep  Bike    850             0.119   0.00    0.00    0.12
21-Sep  Bike    871             0.122   0.00    0.00    0.12
21-Sep  Bike    712             0.100   0.00    0.00    0.10
24-Sep  Bus/Car 381             0.053   2.16    1.42    3.63

Stats

Row Labels  Average of Total CO2 (kg)   StdDev of Total CO2 (kg)    Count
Bike        0.118315556             0.017288929                     9
Bus/Car     3.605146667             0.015794736                     9

Conclusion

The data indicates that the null hypothesis is rejected at p < 0.01.

Discussion

This weakens the OP’s friend’s argument that bicycling causes as much environmental damage as commuting.

The environmental cost of heating the water for a shower was not included, as it seems to have minimal impact (see dancek's fine answer on this point).

Even with a substitution of a more typical American diet with more meats and fewer vegetables, the “bicycling is as bad as a car” hypothesis would be rejected.

4

This paper from the Stockholm University estimates the energy used for the production of a good hamburger is 20 MJ (the high estimate)

http://www.infra.kth.se/fms/pdf/energyuse.pdf

A liter of gasoline contains 35MJ of energy. So two cheeseburgers more per day gives 1L gasoline consumption and a change. Which is usually enough to get 25km in a light car - think Toyota aygo. Add some energy for water heating and moving the water to/from your residence

So things look pretty close. If you replace the meat - things look much better.

Your biggest contribution to saving the environment comes not from the cycling itself, but from not owning a car.

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    Nothing looks close. You’re still going to eat, even if you’re not cycling. Granted, you’ll eat less but how much less? That’s the crux. – Konrad Rudolph Aug 21 '11 at 18:59
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    Well 25 km cycling burns almost a thousand calories which is two big macs. – Daniel Iankov Aug 21 '11 at 19:15
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    You're all 3 orders of magnitude away. The numbers might be about kcal, not cal-values. – user unknown Aug 22 '11 at 1:57
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    Except time I suppose. The only time I have used kiloseconds was when my boss tried to demand detailed time sheets. – Daniel Iankov Aug 23 '11 at 21:10
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    But you don't get 35MJ gasoline for free. How many gasoline do you need to get the gasoline (and bring it to gasoline station)? – user unknown Aug 23 '11 at 22:07

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